The present invention is directed towards the structure of an inline runner of an injection molding machine. The inline runner has opposing series of parallel grooves to provide for mixing of an injected compound as it flows through the runner, uniformly increasing the compound temperature.
Traditionally, hot runners have been used in injection molding to distribute rubber from top or side injection ports to small injection gates located at the entrance to single or multiple injection mold cavities. A traditional hot runner is designed to minimize pressure losses through the runner so that the maximum pressure drop and corresponding temperature increase occurs at the drop gate entrances to an injection mold cavity. Rubber flow through a traditional hot runner design is laminar because the high viscosity of the rubber prevents turbulence. Rubber is instead heated as it flows through the runner by conduction and by shear heating at the boundary layer between the rubber flow profile and the surface of the metal mold. Thus the outside layer of rubber in contact with the mold surface is heated, but the rubber flowing through the center of the runner is not heated uniformly by the mold during the short time that a part is being filled.
Attempts have been made to increase the mixing of a fluid material prior to injection into a mold. U.S. Pat. No. 5,262,119 discloses mixing thermoplastic material by placing static mixers in the flow channels leading to a mold. The static mixers act to disperse degraded material or the wrong color material back into the stream for purposes of uniformity. The static mixer is comprised of a fixed twisted metal blade. U.S. Pat. No. 5,688,462 discloses static mixers as part of the cold runners leading into a mold block. The static mixers are a series of baffles, resembling bent fingers, in the runner about which the thermoplastic must flow.
U.S. Pat. No. 3,924,989 discloses an after-mixer in an apparatus for molding thermoset urethane materials. After the components of the material are combined in a mixing chamber, the material is sent into an after-mixer to ensure complete mixing of the components. One disclosed after-mixer is defined by a series of cross channels, the cross-sectional width of the channels increasing and decreasing.
U.S. Pat. No. 4,027,857 discloses a static mixing nozzle for injection molding thermoplastic. The disclosed four way branch static mixer is employed for improved blending of materials.
None of the disclosed after mixers are suitable for rubber molding, nor do any of the teachings appreciate the use of such mixers in thermoelastic or thermoset rubber molding. In the thermoplastic injection molding methods and apparatus disclosed in the prior art, it is a goal to achieve greater color and compound uniformity in the mixing of the thermoplastic or thermoset urethane flow. Heating of the flow stream to its highest optimum temperature is achieved prior to the introduction of the flow stream into the mixer or after-mixer, and to achieve a molded product, the flow stream is cooled after it is injected into the mold to form a solid article. Thus, while it is desired to maintain the flow stream at a defined temperature, it is not a desire of the prior art to increase the temperature of the flow stream as the flow stream travels toward the mold as greater cooling in the mold would be then required if the flow stream entered the mold at a higher temperature.
The present invention is directed to a method and an assembly for injection molding thermoelastic and thermoset rubbers. The disclosed invention provides for quicker, more efficient, and more uniform heating of the elastomer prior to entering the mold cavity.
The disclosed assembly for injection molding rubber is comprised of a mold cavity, a gate located immediately adjacent to the mold cavity, and a hot runner in communication with the gate. A portion of the hot runner has a first and a second plurality of spaced flow channels disposed at intersecting angles to each other to create cross directional flow of the rubber. The cross directional flow of the rubber provides for heating of the rubber as it flows through the non-linear, non-planar path created for mixing of the rubber along the channels and at the junctions of the cross directional flows; the rubber streams thermally mixing and uniformly increasing the temperature of the rubber.
In one aspect of the invention, the first and second plurality of spaced flow channels are located in a distribution runner adjacent to the gate.
In another aspect of the invention, the first and second plurality of flow channels are inclined at angles of 15xc2x0 to 70xc2x0 relative to the centerline of the runner, preferably 30xc2x0 to 60xc2x0 relative to the centerline of the runner.
In another aspect of the invention, the first and second plurality of flow channels have a cross-sectional configuration selection from among the following shapes of semicircular, elliptical, triangular, trapezoidal, square, polygonal, and curvilinear.
The disclosed method of producing a molded rubber article comprises injecting a rubber into a hot runner, passing the rubber through a mold gate and into a mold cavity. The process is characterized by the rubber flowing at cross angles after the rubber is injected into the runner and before the rubber enters into the mold gate to uniformly increase the temperature of the rubber as it flows through the runner.
In one aspect of the disclosed method, the runner is a branched distribution runner leading to a plurality of molds.